The activation of Factor X by Factor IXa in the presence of Factor VIIICa, phospholipid (or platelets) and calcium will be studied in a procine system. Recently developed methods for purifying porcine Factor VIIIC to apparent homogeneity make a rigorous biochemical analysis of this system possible for the first time. An experimental approach is outlined to explore the equilibrium interactions of the various components of the system and the kinetics of the overall reaction. The experimental approach relies heavily upon past experience with the prothrombinase system which may be very similar to intrinsic pathway Factor X activation. This study will contribute to the understanding of the biology of Factor VIII, the Factor X activation in general, and will provide necessary data for the analysis of the possible interaction of the intrinsic pathway X activator complex and the prothrombinase complex.